The present invention relates generally to on board inert gas generating systems, and more particularly, to an on board inert gas generating system having a fast warm up feature. Even more particularly, the present invention relates to a method and apparatus for quickly warming up a permeable membrane air separation system.
On Board Inert Gas Generating Systems (OBIGGS) Air Separation Modules (ASM) utilizing molecular sieve employing a pressure swing adsorption (PSA) process have been used for many years to inert the fuel tanks on aircraft such as the AH-64 helicopter, C-17 transport and the V-22 tilt-Rotor aircraft. During normal operational modes, this PSA technology uses conditioned engine bleed air at typical operating temperatures that may range from xe2x88x9260 to 130 degrees F., and removes oxygen to generate nitrogen enriched air (NEA). This wide temperature range has a relatively minor impact on the performance of the PSA technology. The NEA product gas is used to purge the ullage space above the fuel in the fuel tanks which is initially filled with air containing oxygen at the normal concentrations of about 21% oxygen. Reducing the oxygen concentration to or below about 9% oxygen in the ullage space above the fuel in the fuel tank on the aircraft eliminates the potential for an explosion when the fuel tank is exposed to potential ignition sources such as electrical sparks or incendiary rounds. During normal aircraft start-ups the initial air temperatures received by the PSA OBIGGS can be very cold, depending upon the existing environmental conditions and how quickly warm engine bleed air is provided to the OBIGGS. The performance of OBIGGS employing the PSA technology is affected relatively little by air temperature of xe2x88x9260 to +130 degrees, which results in immediate, efficient generation of NEA. No warm-up or start-up time is needed.
A newer OBIGGS ASM technology is now being employed to generate NEA on some aircraft such as the USAF F-22, and will be employed on future aircraft such as the JSF. This newer technology utilizes hollow fibers of permeable membrane (PM) which operate most efficiently at operating temperatures of about +140 to +220 degrees F. The inlet supply air is introduced to the inside of one end of a bundle of thousands of small hollow fibers, and oxygen permeates through the walls and is exhausted. The nitrogen permeates through the hollow fibers much slower than oxygen, such that at the opposite end of the hollow fibers NEA is generated by the loss of oxygen via permeation through the permeable membrane fiber as the source gas traverses the length of each hollow fiber. The PM technology has the potential advantage of exhibiting good NEA generation rates at higher operating temperatures than the PSA OBIGGS technology. This can be particularly attractive for some applications where the preferred source of air is obtained from the aircraft engine bleed air, which is generally warmer than +220 degrees F. Less cooling of the air would be required to use the PM technology than the PSA OBIGGS technology. This reduces heat exchanger size and weight, which is an important benefit for aircraft applications.
All OBIGGS ASMs that employ either PSA or PM technologies utilize a flow control device to regulate the amount of NEA allowed to flow from the OBIGGS ASM. This flow control device is usually a fixed orifice that is sized to limit the NEA flow rate such that the oxygen content in the NEA product gas is limited to about 9% or less. Both PSA and PM product gas nitrogen purity is inversely proportional to the NEA product flow from the ASM. High NEA flow through the fibers allows less time for oxygen to permeate through the PM fiber which results in the NEA oxygen content to increase. A flow rate that is too high can result in the fuel tanks having explosive combinations of oxygen and fuel vapors. Too little flow results in the desired low-oxygen content, but the low NEA flow rate can significantly delay the purging of the oxygen-laden air from the fuel tank and delay the achievement of a safe inert condition.
As noted earlier PM ASMs operate best at higher operating temperatures of +140 to +220 degrees F. When aircraft are required to xe2x80x9cscramblexe2x80x9d and be airborne rapidly, the time to inert the fuel tanks to a safe condition quickly is important. Waiting for the PM to reach the higher normal PM operating temperature results in a delay in reaching optimum performance and reduces the NEA generation rate available during this start-up or warm-up period. The PM must be warmed up quickly or longer times to inert the fuel tanks will be required. Some fighter aircraft missions require the aircraft to be airborne in five minutes or less; however, it can take longer than that just to warm up the PM ASM and attain near normal performance. Thus, a need exists in the art for a system capable of warming up a PM ASM in five minutes or less.
It is, therefore, an object of the present invention to reduce the time required to warm up a PM ASM.
Another object of the present invention is to provide a PM ASM system which provides warm inlet air from the engine bleed air system into the PM ASM.
Another object of the present invention is to initially increase NEA flow through the PM ASM to speed up the warming of the hollow fibers in the PM.
Yet another object of the present invention is to provide a method and apparatus that directs a portion of the warm engine bleed air into the outer shell of the PM ASM.
Still another object of the present invention is to provide a method and apparatus which can utilize NEA product flow and/or engine bleed air to warm the hollow fibers of the PM separator.
Yet another object of the present invention is to provide NEA product gas having an oxygen content of about 9% or less in less than five minutes.
Another object of the present invention is to provide a permeable membrane air separation system that can be warmed up in less than five minutes.
Advantageously, the present invention is directed to an apparatus and method for warming up the PM ASM to obtain near normal performance in approximately three minutes. This compares to present performance of approximately 7.5 minutes.
The present invention uses a fast start valve that allows an increased flow of NEA through the PM ASM that is then directed into the outer shell of the PM ASM. This increased flow of warm inlet air from the engine bleed air system into, through around each hollow fiber, and accelerates the warming of the hollow fibers in the PM ASM. An alternative approach accelerates PM ASM warm up by directing a portion of the warm inlet air directly into the outer shell of the PM ASM while the normal amount of NEA is allowed to flow through the inside of the PM fibers. A third approach is to direct a combination of increased NEA flow and inlet airflow into the outer shell to accelerate warming of the PM ASM.
These and other objects of the present invention are achieved in an air separation module assembly having fast warm up capability. The air separation module is connected to a source of warm air. An air separation module has an inlet, an outlet, an exit port and an entry point. The inlet is connected to the source of warm air. A valve is connected to the entry point. The valve is also connected to at least one of the exit port and the source of warm air.
The foregoing objects of the present invention are achieved by a method of quickly warming up an air separation module. Warm air is flowed through an air separation module to separate nitrogen gas therefrom. A portion of the separated nitrogen gas is directed to an ullage space of a fuel tank. A portion of the separated nitrogen gas is directed back into the air separation module.
The foregoing and other objects of the present invention are achieved by a method of quickly warming up an air separation module. Warm air is flowed through an air separation module to separate nitrogen gas therefrom. A portion of the separated nitrogen gas is directed to an ullage space of a fuel tank. A portion of the separated nitrogen gas is directed back into the air separation module. A source of warm air is directed into an entry point of an air separation module. The warm air is then exhausted from the air separation module.
The foregoing objects of the present invention are achieved by a method of quickly warming up an air separation module. Warm air is flowed through an air separation module to separate nitrogen gas therefrom. A portion of the separated nitrogen gas is directed to an ullage space of a fuel tank. A portion of the separated nitrogen gas is directed back into the air separation module. Air is directed from a source of warm air into an entry point of an air separation module and exhausts the warm air from an exit port of the air separation module.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.